Imaging optical instrument

ABSTRACT

An imaging optical instrument causes light emitted from a plurality of light sources arranged adjacent an object to image on recording surfaces of a photosensitive or thermosensitive material. The optical instrument includes a two-sided telecentric optical system having a first lens disposed adjacent an entrance end of a lens barrel, and a second lens disposed adjacent an exit end of the lens barrel, and an aperture stop disposed adjacent a meeting point of a rearward focus of the first lens and a forward focus of the second lens. The aperture stop has an aperture formed centrally thereof for allowing passage of beams that should contribute to image formation, and a refracting portion surrounding the aperture for refracting beams that should be intercepted and causing these beams to leave the lens barrel from the exit end thereof.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an imaging optical instrumentfor imaging light from an object on a recording surface.

[0003] 2. Description of the Related Art

[0004] Generally, such an imaging optical instrument includes anaperture stop disposed in an optical system for limiting beamscontributing to image formation, in order to secure excellent imagingperformance or to secure a required focal depth. Such an aperture stopis formed of a light-shielding sheet of metal or plastic defining acircular or polygonal aperture. The aperture stop is fixed inside asealed space defined by lenses and a barrel holding the lenses.

[0005] Where, for example, high power lasers are used as light sourcesto emit light for forming images on a recording surface, light of highenergy level enters the optical instrument from the objects. In such acase, the aperture stop is heated to a high temperature as a result ofshielding the high energy light.

[0006] The heat of the aperture stop reaches the lens barrel by thermalconduction, and may deform the lens barrel locally. Such a deformationdisrupts a proper positional relationship between the lenses to lowerimaging performance. The heat of the aperture stop filling the lensbarrel interior by thermal convection heats up and expands the entirelens barrel. This results in an imaging position shifting along theoptical axis. With a further increase in the temperature of the aperturestop, the aperture stop itself may melt or vaporize to lose itslight-shielding function. The shielding material may adhere to lenssurfaces to lower optical efficiency also.

[0007] It is conceivable to deal with this problem by arranging for theaperture stop to be held independently of the lens barrel. Where such aconstruction is employed, surfaces of the lenses arranged forwardly andrearwardly the aperture stop are exposed to atmosphere outside the lensbarrel. Consequently, dust and the like present in the atmosphere couldadhere to the lens surfaces to lower optical efficiency.

SUMMARY OF THE INVENTION

[0008] The object of the present invention, therefore, is to provide animaging optical instrument capable of effectively preventing an increasein the temperature of an aperture stop.

[0009] The above object is fulfilled, according to the presentinvention, by an imaging optical instrument for causing light incidentat an entrance end of a lens barrel to pass through an aperture stop andthereafter to leave the lens barrel from an exit end thereof, therebyimaging on a recording surface, wherein the aperture stop comprises anoptical element having a passage portion formed centrally thereof forallowing passage of beams that should contribute to image formation, anda refracting portion surrounding the passage portion for refractingbeams that should be intercepted and causing the beams that should beintercepted to leave the lens barrel from the exit end.

[0010] This imaging optical instrument is capable of effectivelypreventing an increase in the temperature of the aperture stop and lensbarrel.

[0011] In a preferred embodiment of the invention, the aperture stopcomprises an optical element having an aperture formed centrally thereoffor allowing passage of beams that should contribute to image formation,and a refracting portion surrounding the aperture for refracting beamsthat should be intercepted and causing the beams that should beintercepted to leave the lens barrel from the exit end. That is, theaperture formed in the optical element is used as the passage portionfor allowing passage of the beams.

[0012] In another preferred embodiment, the aperture stop comprises anoptical element having a parallel plate portion formed centrally thereoffor allowing passage of beams that should contribute to image formation,and a refracting portion surrounding the parallel plate portion forrefracting beams that should be intercepted and causing the beams thatshould be intercepted to leave the lens barrel from the exit end. Thatis, the parallel plate portion formed in the optical element is used asthe passage portion for allowing passage of the beams.

[0013] In another aspect of the invention, there is provided an imagingoptical instrument for causing light incident at an entrance end of alens barrel to pass through an aperture stop and thereafter to leave thelens barrel from an exit end thereof, thereby imaging on a recordingsurface, wherein the aperture stop comprises an optical element havingan aperture formed centrally thereof for allowing passage of beams thatshould contribute to image formation, and a reflecting portionsurrounding the aperture for reflecting beams that should be interceptedand causing the beams that should be intercepted to leave the lensbarrel from the entrance end.

[0014] In a further aspect of the invention, there is provided animaging optical instrument for causing light incident at an entrance endof a lens barrel to pass through an aperture stop and thereafter toleave the lens barrel from an exit end thereof, thereby imaging on arecording surface, wherein the aperture stop comprises a plurality oflight-shielding plates arranged at predetermined intervals along anoptical axis of the lens barrel and defining apertures with diametersthereof varying stepwise, the lens barrel defining a vent hole adjacentthe aperture stop.

[0015] Other features and advantages of the present invention will beapparent from the following detailed description of the embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] For the purpose of illustrating the invention, there are shown inthe drawings several forms which are presently preferred, it beingunderstood, however, that the invention is not limited to the precisearrangement and instrumentalities shown.

[0017]FIG. 1 is a sectional side view of an imaging optical instrumentin a first embodiment of the invention;

[0018]FIG. 2 is an explanatory view of aperture stops;

[0019]FIG. 3 is a sectional side view of an imaging optical instrumentin a second embodiment of the invention;

[0020]FIG. 4 is a sectional side view of an imaging optical instrumentin a third embodiment of the invention; and

[0021]FIG. 5 is an exploded perspective view of an aperture stop in thethird embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] An embodiment of the present invention will be describedhereinafter with reference to the drawings. FIG. 1 is a sectional sideview of an imaging optical instrument in a first embodiment of theinvention.

[0023] This imaging optical instrument is designed for causing lightemitted from a plurality of light sources 2 arranged adjacent an objectto image on a recording surfaces 8 of a photosensitive orthermosensitive material. The imaging optical instrument includes a lensbarrel 3 having a lens 4 disposed adjacent an entrance end of the lensbarrel 3 and a lens 5 disposed adjacent an exit end of the lens barrel 3to act as a two-sided telecentric optical system 6, and an aperture stop7. This imaging optical instrument is used, for example, in an imagerecording apparatus that records images by scanning the photosensitiveor thermosensitive material with beams modulated by image signals.

[0024] The light sources 2 are formed of semiconductor lasers or lightemitting diodes, for example. Further, the light sources 2 may be in theform of ends of a plurality of optical fibers connected to semiconductorlasers or light emitting diodes, or a group of openings illuminated bysemiconductor lasers or light emitting diodes. These light sources 2 arearranged to emit light having intensity gravities running parallel toone another, and preferably parallel to the optical axis of thetwo-sided telecentric optical system 6.

[0025] While a plurality of light sources 2 are used in the aboveembodiment, this is not absolutely necessary. A single light source 2may be used instead.

[0026] The two-sided telecentric optical system 6 includes, in thesimplest form, two positive lenses 4 and 5 spaced from each other by asum of focal lengths thereof. The arrangement of the light sources 2 ismagnified or reduced by the two-sided telecentric optical system 6 intoan arrangement of luminous points for recording images with a desiredresolution on the recording surface 8. Since the principal rays in thebeams emitted from the light sources 2 are parallel to one another, thearrangement of luminous points remains constant even when the recordingsurface 8 deviates forward or rearward from an imaging plane. Thus,images may be recorded with high precision.

[0027] The aperture stop 7 is disposed adjacent a meeting point of arearward focus of the lens 4 and a forward focus of the lens 5.

[0028]FIG. 2 is an explanatory view schematically showing variousaperture stops 7 a, 7 b and 7 c (which may be referred to collectivelyas “aperture stop 7”). Each of FIGS. 2(a), 2(b) and 2(c) includes a sideview at left and a front view at right. FIGS. 2(a), 2(b) and 2(c) showvarious configurations with exaggeration.

[0029] The aperture stop 7 shown in FIG. 1 corresponds to the aperturestop 7 a shown in FIG. 2(a). The aperture stop 7 a is formed by drillinga plano-convex lens as shown in phantom in the side view at the left ofFIG. 2(a). The aperture stop 7 a has an aperture 72 formed centrallythereof to act as a passage of light, and a refracting portion 71surrounding the aperture 72 to refract the light. The aperture 72 has aninside diameter corresponding to an aperture diameter required of theaperture stop 7 a. This aperture diameter determines an NA (number ofapertures) of the beams used in recording images. The aperture diameteris determined by taking optical efficiency and focal depth into account.

[0030] In the imaging optical instrument having the above construction,of the beams emitted from the light sources 2, the parts that shouldcontribute to image formation pass straight through the aperture 72 ofthe aperture stop 7 a. Consequently, the two-sided telecentric opticalsystem 6 causes these beams to form images of the light sources 2 on therecording surface 8.

[0031] On the other hand, of the beams emitted from the light sources 2,the parts that should be intercepted (i.e. the parts that should notcontribute to image formation) are refracted by the refracting portion71 of the aperture stop 7 a. As shown in hatching in FIG. 1, such partsof the beams, after exiting the lens barrel 3 reach around the imagingpoints which are formed by the beams having passed through the aperture72 of the aperture stop 7 a without forming images and without beingcondensed. By appropriately shaping the refracting portion 71 of theaperture stop 7 a, the density of light energy in these areas may bemade far less than the density of light energy at the imaging point onthe recording surface 8 of the light having passed through the aperture72 of the aperture stop 7 a. Thus, an image is recorded without beingaffected by the light reaching the areas around the imaging pointwithout forming images and without being condensed.

[0032] The aperture stop 7 a allows the unwanted light to leave the lensbarrel 3 from the exit end thereof instead of intercepting such unwantedlight. Consequently, this imaging optical instrument effectively avoidsthe generation of heat occurring in the prior art.

[0033] The shape of refracting portion 71 of the aperture stop 7 a (i.e.the shape of the plano-convex lens used as stock) must be such that thebeams thereby refracted do not impinge on inner walls of the lens barrel3, but reach the areas around the imaging point on the recording surface8 without forming images and without being condensed. Where therefracting portion 71 has an excessively low level of refractive power,the beams having passed through the refracting portion 71 would reachsmall areas around the imaging point on the recording surface 8. Thedensity of light energy in these areas is not sufficiently lowered,thereby to produce an adverse effect on image recording. A permissiblelevel of light energy of the beams passing through the refractingportion 71 for producing no adverse effect on image recording isdependent chiefly on the characteristics of a recording medium set tothe recording surface 8. On the other hand, where the refracting portion71 has an excessively high level of refractive power, the beams havingpassed through the refracting portion 71 would impinge on the innerwalls of the lens barrel 3 to be absorbed and converted to heat by theinner walls. This gives rise to such problems as deterioration inimaging performance and displacement of an imaging position.

[0034] A high degree of profile irregularity is not required of therefracting portion 71 of the aperture stop 7 a, which allows theaperture stop 7 a to be manufactured by using an inexpensiveplano-convex lens or the like. At this time, preferably, a largeaberration is provided in order to reduce the energy density of thebeams reaching areas around an imaging point without forming images andwithout being condensed.

[0035] The above aperture stop 7 a may be replaced by the aperture stop7 b shown in FIG. 2(b) or the aperture stop 7 c shown in FIG. 2(c).

[0036] The aperture stop 7 b shown in FIG. 2(b) is formed by drilling acone prism as shown in phantom in the side view at the left of FIG.2(b). The aperture stop 7 b has an aperture 75 formed centrally thereof,and a refracting portion 74 surrounding the aperture 75 to refract thelight. The aperture 75 has an inside diameter corresponding to anaperture diameter required of the aperture stop 7 b.

[0037] When this aperture stop 7 b is used, as when the above aperturestop 7 a, is used, of the beams emitted from the light sources 2, theparts that should contribute to image formation pass straight throughthe aperture 75 of the aperture stop 7 b. Consequently, the two-sidedtelecentric optical system 6 causes these beams to form images of thelight sources 2 on the recording surface 8. On the other hand, of thebeams emitted from the light sources 2, the parts that should beintercepted are refracted by the refracting portion 74 of the aperturestop 7 b. Such parts of the beams reach areas around the imaging pointwithout forming images and without being condensed.

[0038] In the two-sided telecentric optical system 6 shown in FIG. 1,the beams are in the state of nearly parallel beams where the aperturestop 7 is installed. Thus, when the aperture stop 7 b obtained bydrilling a cone prism is used, variations in the incident angle and exitangle may be reduced. This facilitates optimization of an antireflectioncoating usually applied to this type of optical element, and assures animproved antireflection effect. Consequently, heat generation adjacentthe aperture stop 7 b may be more effectively suppressed than where theforegoing aperture stop 7 a is used.

[0039] The aperture stop 7 c shown in FIG. 2(c) is formed by cutting andpolishing a surface area 79 of a plano-convex lens as shown in phantomin the side view at the left FIG. 2(c). The aperture stop 7 c has aparallel plate portion 78 formed centrally thereof to act as a lightpassage, and a refracting portion 77 surrounding the parallel plateportion 78 to refract the light. The parallel plate portion 78 has adiameter corresponding to an aperture diameter required of the aperturestop 7 c.

[0040] When this aperture stop 7 c is used, as when the above aperturestop 7 a or 7 b is used, of the beams emitted from the light sources 2,the parts that should contribute to image formation pass straightthrough the parallel plate portion 78 of the aperture stop 7 c.Consequently, the two-sided telecentric optical system 6 causes thesebeams to form images of the light sources 2 on the recording surface 8.On the other hand, of the beams emitted from the light sources 2, theparts that should be intercepted are refracted by the refracting portion77 of the aperture stop 7 c. Such parts of the beams reach areas aroundthe imaging point without forming images and without being condensed.

[0041] With the foregoing aperture stops 7 a and 7 b, parts of the beamsemitted from light sources 2 arranged off the optical axis could enterthe refracting portions 71 and 74 through inner walls of apertures 72and 75 to be absorbed, scattered or reflected. This would cause atemperature increase adjacent the aperture stops 7 a and 7 b, or lowerimaging performance because of flares. The aperture stop 7 c is freefrom such problems. However, when this aperture stop 7 c is used, theentire system including the parallel plate portion 78 must be designedwith aberration corrections.

[0042] In the above embodiment, a plano-convex lens or prism is used forthe aperture stop 7. In place of the plano-convex lens or prism, aflattened lens such as a Fresnel lens or graded index lens may be used.Instead of the lens, a diffracting optical element may be used.

[0043] Another embodiment of the invention will be described next. FIG.3 is a sectional side view of an imaging optical instrument in a secondembodiment. Like reference numerals are used to identify like partswhich are the same as in the first embodiment and will not be describedagain.

[0044] The foregoing imaging optical instrument in the first embodimentrefracts the beams that should be intercepted and allows these beams toleave the lens barrel 3 from the exit end thereof. The imaging opticalinstrument in the second embodiment reflects the beams that should beintercepted and allows these beams to leave the lens barrel 3 from theentrance end thereof. This imaging optical instrument employs anaperture stop 7 similar to the aperture stop 7 a shown in FIG. 2(a), andformed of a plano-convex lens having a central aperture, but with areflective coating 70 applied to the convex surface thereof (opposed tothe recording surface 8 shown in FIG. 3). The aperture of this aperturestop 7 has an inside diameter corresponding to the aperture diameterrequired of the aperture stop 7.

[0045] In the imaging optical instrument in the second embodiment, ofthe beams emitted from the light sources 2, the parts that shouldcontribute to image formation pass straight through the aperture of theaperture stop 7. Consequently, the two-sided telecentric optical system6 causes these beams to form images of the light sources 2 on therecording surface 8.

[0046] On the other hand, of the beams emitted from the light sources 2,the parts that should be intercepted are reflected by the reflectivecoating 70 on the aperture stop 7. As shown in hatching in FIG. 3, suchparts of the beams leave the lens barrel 3 from the entrance endthereof.

[0047] In the imaging optical instrument in the second embodiment, as inthe imaging optical instrument in the first embodiment, the aperturestop 7 allows the unwanted light to leave the lens barrel 3, from theentrance end thereof in this embodiment, instead of intercepting suchlight. Consequently, this imaging optical instrument effectively avoidsthe generation of heat occurring in the prior art.

[0048] The shape of reflective coating 70 on the aperture stop 7 (i.e.the shape of the plano-convex lens used as stock) must be such that thebeams thereby reflected do not impinge on the inner walls of the lensbarrel 3, but exit from the entrance end. In this embodiment, aplano-convex lens is used as stock for forming the aperture stop 7 todetermine the shape of reflective coating 70. An element other than alens may be used. Instead of using the plano-convex lens with reflectivecoating 70, a plane mirror defining a central opening may be used.

[0049] A further embodiment of the invention will be described next.FIG. 4 is a sectional side view of an imaging optical instrument in athird embodiment. FIG. 5 is an exploded perspective view of an aperturestop in the third embodiment. Like reference numerals are used toidentify like parts which are the same as in the first and secondembodiments and will not be described again.

[0050] The imaging optical instruments in the first and secondembodiments described hereinbefore cause the beams that should beintercepted to leave the lens barrel 3 from the exit or entrance endthereof. The imaging optical instrument in the third embodiment isconstructed to shield and absorb the beams that should be intercepted.

[0051] Specifically, an aperture stop 9 employed in the imaging opticalinstrument in the third embodiment has five thin light-shielding plates91, 92, 93, 94 and 95 arranged, with spacers 96 in between, along theoptical axis of the lens barrel 3. The five light-shielding plates 91,92, 93, 94 and 95 define apertures having aperture diameters D1, D2, D3,D4 and D5, respectively. The aperture diameters D1, D2, D3, D4 and D5are progressively smaller from the light-shielding plate 91 to thelight-shielding plate 95. The diameter D5 of the aperture in thelight-shielding plate 95 corresponds to an aperture diameter required ofthe aperture stop 9.

[0052] The lens barrel 3 has an opening formed adjacent the aperturestop 7, with a filter 31 fitted in the opening for removing foreignmatters. The lens barrel 3 has a further opening formed in a positionopposed to the above opening, with an exhaust fan 32 fitted therein.

[0053] With the aperture stop 9 in this embodiment, each of thelight-shielding plates 91, 92, 93, 94 and 95 bears its share inshielding the beams emitted from the light sources 2. This effectivelyavoids a sharp increase in the temperature of the light-shielding plates91, 92, 93, 94 and 95. The light-shielding plates 91, 92, 93, 94 and 95are arranged with the spacers 96 in between, and these spacers 96 have aheat radiating effect comparable to that of heat radiating fins. Thus,the light-shielding plates 91, 92, 93, 94 and 95 are prevented frombeing heated to a high temperature.

[0054] Air ascending in the lens barrel 3 as a result of heat radiationfrom the light-shielding plates 91, 92, 93, 94 and 95 is discharged fromthe lens barrel 3 by the exhaust fan 32. Fresh air is supplied into thelens barrel 3 through the filter 31 that removes foreign matters. Thisprevents the interior of the lens barrel 3 from becoming heated to ahigh temperature. The filter 31 for removing foreign matters effectivelyprevents dust and the like from entering the lens barrel 3.

[0055] In each of the embodiments described hereinbefore, the imagingoptical instrument according to the invention is applied to an imagerecording apparatus. However, the imaging optical instrument accordingto the invention is not limited to such application.

[0056] The present invention may be embodied in other specific formswithout departing from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

[0057] This application claims priority benefit under 35 U.S.C. Section119 of Japanese Patent Application No.2001-142585 filed in the JapanesePatent Office on May 14, 2001, the entire disclosure of which isincorporated herein by reference.

What is claimed is:
 1. An imaging optical instrument for causing lightincident at an entrance end of a lens barrel to pass through an aperturestop and thereafter to leave the lens barrel from an exit end thereof,thereby imaging on a recording surface, wherein said aperture stopcomprises an optical element having a passage portion formed centrallythereof for allowing passage of beams that should contribute to imageformation, and a refracting portion surrounding said passage portion forrefracting beams that should be intercepted and causing said beams thatshould be intercepted to leave said lens barrel from said exit end. 2.An imaging optical instrument as defined in claim 1, wherein saidaperture stop comprises an optical element having an aperture formedcentrally thereof for allowing passage of beams that should contributeto image formation, and a refracting portion surrounding said aperturefor refracting beams that should be intercepted and causing said beamsthat should be intercepted to leave said lens barrel from said exit end.3. An imaging optical instrument as defined in claim 2, furthercomprising a two-sided telecentric optical system including a first lensdisposed adjacent said entrance end of said lens barrel, and a secondlens disposed adjacent said exit end of said lens barrel, said aperturestop being disposed adjacent a meeting point of a rearward focus of thefirst lens and a forward focus of the second lens.
 4. An imaging opticalinstrument as defined in claim 3, wherein said optical element is a lenshaving an aperture formed centrally thereof.
 5. An imaging opticalinstrument as defined in claim 3, wherein said optical element is aprism having an aperture formed centrally thereof.
 6. An imaging opticalinstrument as defined in claim 1, wherein said aperture stop comprisesan optical element having a parallel plate portion formed centrallythereof for allowing passage of beams that should contribute to imageformation, and a refracting portion surrounding said parallel plateportion for refracting beams that should be intercepted and causing saidbeams that should be intercepted to leave said lens barrel from saidexit end.
 7. An imaging optical instrument as defined in claim 6,further comprising a two-sided telecentric optical system including afirst lens disposed adjacent said entrance end of said lens barrel, anda second lens disposed adjacent said exit end of said lens barrel, saidaperture stop being disposed adjacent a meeting point of a rearwardfocus of the first lens and a forward focus of the second lens.
 8. Animaging optical instrument for causing light incident at an entrance endof a lens barrel to pass through an aperture stop and thereafter toleave the lens barrel from an exit end thereof, thereby imaging on arecording surface, wherein said aperture stop comprises an opticalelement having an aperture formed centrally thereof for allowing passageof beams that should contribute to image formation, and a reflectingportion surrounding said aperture for reflecting beams that should beintercepted and causing said beams that should be intercepted to leavesaid lens barrel from said entrance end.
 9. An imaging opticalinstrument as defined in claim 8, further comprising a two-sidedtelecentric optical system including a first lens disposed adjacent saidentrance end of said lens barrel, and a second lens disposed adjacentsaid exit end of said lens barrel, said aperture stop being disposedadjacent a meeting point of a rearward focus of the first lens and aforward focus of the second lens.
 10. An imaging optical instrument forcausing light incident at an entrance end of a lens barrel to passthrough an aperture stop and thereafter to leave the lens barrel from anexit end thereof, thereby imaging on a recording surface, wherein saidaperture stop comprises a plurality of light-shielding plates arrangedat predetermined intervals along an optical axis of said lens barrel anddefining apertures with diameters thereof varying stepwise, said lensbarrel defining a vent hole adjacent said aperture stop.
 11. An imagingoptical instrument as defined in claim 10, further comprising atwo-sided telecentric optical system including a first lens disposedadjacent said entrance end of said lens barrel, and a second lensdisposed adjacent said exit end of said lens barrel, said aperture stopbeing disposed adjacent a meeting point of a rearward focus of the firstlens and a forward focus of the second lens.
 12. An imaging opticalinstrument as defined in claim 11, further comprising forced ventilationmeans mounted in said vent hole.
 13. An imaging optical instrument asdefined in claim 11, further comprising a filter mounted in said venthole for removing foreign matters.